A serious problem is frequently encountered in coal and oil fired boilers and this is often more serious in boilers of large size such as those which are used in electricity generating stations. This problem is two-fold:
(a) Erosion of boiler tubes by fly ash carried by the combustion gases used to heat the boiler tubes takes place and increased temperature accentuates such erosion.
(b) Corrosion of the boiler tubes takes place resulting from the presence of corrosive elements in the fuel, in particular sulphur, chlorine and vanadium. On combustion of the fuel, these elements form corrosive products which corrode the metal surface of the boiler tubes.
In some areas, notably superheater zones, corrosive slags may deposit on the tubes. This also results in corrosion of the tubes.
Such erosion and corrosion can result in very high replacement costs not only in the costs of the tubes themselves but also in the costs associated with downtime of the boiler.
It will of course also be appreciated that the poorer the quality of the coal available for firing the boiler, the higher the erosive and corrosive character of the ash, and the greater is the problem.
Many attempts have been made to find an economical solution to this problem but, to the applicant's knowledge, no satisfactory solution has been achieved. In the United Kingdom the problem is mainly a corrosive one. Extensive research work has been conducted there by the Central Electricity Generating Board but the problem remains unresolved. The closest to any form of success that has been achieved, as far as the applicant is aware, is the use of an expensive technique whereby corrosion resistant metal alloys are co-extruded over mild steel tubes in which case costs are five to fifteen times higher than for mild steel tubes.
Thermally sprayed metal and ceramic coatings have not proved successful due to their inherent porosity. This permits the ingress of corrosive gases resulting in the formation of corrosion products at the interface between the tube and coating. This often results in the coating being lifted off by substrate oxidation, and this problem is aggravated by the build-up of slag.
The applicant has now achieved remarkably surprising results by employing a process whereby such subcoating corrosion is at least inhibited.